well, the iro assume that your irrigation system was not installed by asshats. its a bold assumption, yes, but:
if you know the accumulation rate of a head,
assume that you have good head to head coverage (proper overlap and similar arcs, sqft/head responsiblity)
all heads in the zone provide the same flow rate.
not mixing misters and rotors in the same zone
then the controller can make the assumption that "accumulation rate of your head" / "desired accumulation rate" = minutes of runtime (over simplified i know).
for me, it gave me a solid starting point. after 7 years of tweaking my hunter pro c, the iro controller recommend 1 extra minute of runtime on my zones, and 3 extra minutes of soak/rest. that was pretty damned closed to my 7 years of tweaks.
more specifically, "It also doesn't know if I'm using one head to water 100 sq ft in at 30 degree arc, thus the head is going back and forth across the zone quickly and throwing down more water per sq ft per minute, vs one head doing a 270 degree arc on a 1000 sq ft zone and taking a long time to get across it, thus, not throwing down as much water per sq ft per minute."
this is improper irrigation design, more heads should be added to the zone, or the different arcs should be in a different zone. each zone should in my opinion should have heads that are homogeneous. unless you are doing a drip system, you will always end up with too much/little water in areas of a zone.